Normal control of breathing is a complex process that involves, in part, the body's interpretation and response to chemical stimuli such as carbon dioxide, pH and oxygen levels in blood, tissues and the brain. Normal breathing control is also affected by other factors such as wakefulness (i.e., whether the patient is awake or sleeping), emotion, posture and vocalization. Within the brain medulla, there are respiratory control centers that interpret various feed-forward and feed-back signals that affect respiration by issuing commands to the muscles that perform the work of breathing. Key muscle groups are located in the abdomen, diaphragm, pharynx and thorax. Sensors located centrally and peripherally then provide input to the brain's central respiration control areas that enables response to changing metabolic requirements.
For example, ventilation sufficient to meet the body's metabolic needs is maintained primarily by the body's rapid response to changes in carbon dioxide (CO2) levels. Increased CO2 levels (hypercapnia) signal the body to increase breathing rate and depth, resulting in higher blood oxygen levels and subsequent lower blood CO2 levels. Conversely, low CO2 levels (hypocapnia) can result in periods of hypopnea (decreased breathing) or, in the extreme case, apnea (no breathing) since the stimulation to breathe is diminished.
There are many diseases in which loss of normal breathing control is a primary or secondary feature of the disease. Examples of diseases with a primary loss of breathing control are sleep apneas (central, mixed or obstructive; where the breathing repeatedly stops for 10 to 60 seconds) and congenital central hypoventilation syndrome. Secondary loss of breathing control may be due to chronic cardio-pulmonary diseases (e.g., heart failure, chronic bronchitis, emphysema, and impending respiratory failure), excessive weight (e.g., obesity-hypoventilation syndrome), certain drugs (e.g., anesthetics, sedatives, sleeping aids, anxiolytics, hypnotics, alcohol, and narcotic analgesics and/or factors that affect the neurological system (e.g., stroke, tumor, trauma, radiation damage, and ALS). In chronic obstructive pulmonary diseases where the body is exposed to chronically high levels of carbon dioxide, the body adapts to the respiratory acidosis (lower pH) by a kidney mediated retention of bicarbonate, which has the effect of partially neutralizing the CO2/pH respiratory stimulation. Thus, the patient is unable to mount a normal ventilatory response to changes in metabolic demand.
Sleep disordered breathing is an example of where abnormalities in the control of breathing lead to a serious and prevalent disease in humans. Sleep apnea is characterized by frequent periods of no or partial breathing. Key factors that contribute to these apneas include anatomical factors (e.g., obesity), decreased hypercapnic and hypoxic ventilatory responses (e.g., decreased response to high carbon dioxide and low oxygen levels, respectively) and loss of “wakefulness” (respiratory drive to pharyngeal dilator muscles during sleep). Apneic events result in intermittent hypoxia (and the associated oxidative stress) and eventually severe cardiovascular consequences (high blood pressure, stroke, heart attack).
Estimates for U.S. individuals afflicted with conditions wherein there is compromised respiratory control include sleep apneas (15-20 millions); obesity-hypoventilation syndrome (3-5 millions); chronic heart disease (5 millions); chronic obstructive pulmonary disease (COPD)/chronic bronchitis (10 millions); drug-induced hypoventilation (2-10 millions); and mechanical ventilation weaning (0.5 million).
There is a need in the art for novel compounds useful for restoring all or part of the body's normal breathing control system in response to changes in CO2 and/or oxygen levels, with minimal side effects. Further, there is a need in the art for novel compounds that are useful for restoring all or part of the body's normal breathing control system and possess suitable pharmacokinetic properties, such as oral bioavailability. Further, there is a need in the art for novel compounds that are useful for restoring all or part of the body's normal breathing control system and may be administered orally and used in a chronic or acute manner. Further, there is a need in the art for novel compounds that are useful for restoring all or part of the body's normal breathing control system and may be administered parenterally (e.g., intravenously) and used in an acute manner. The present invention addresses and meets these needs.